It is known to deposit thin amorphous hydrogenated carbon films, designated a-C:H, or of a polycrystalline nature on glass or silicon substrates using plasma-assisted chemical vapour phase deposition (PECVD). The gas used for carbon deposition is essentially a mixture of methane and hydrogen. In this connection reference is made to the following articles:
"Electrical and chemical characterization of a-C:H prepared by RF glow discharge", W. J. Varhue et al--J. Appl. Phys. 67(88)--15 Apr. 1990, pp 3835-3841; PA1 "Diamond and diamond-like films: Deposition processes and properties", C. V. Deshpandey et al, J. Vac. Sci. Technol. A7(3), May/June 1989, pp 2294-2302. PA1 A) producing photosensitive resin patterns on the substrate outside the areas to be passivated, PA1 B) subjecting the structure obtained in A) to the action of a radio-frequency plasma essentially constituted by hydrocarbon and thus deposit an amorphous hydrogenated carbon layer on said structure and PA1 C) dissolving the resin patterns in order to eliminate the amorphous carbon deposited on the resin, the amorphous carbon deposited on said areas constituting the said passivation. PA1 Handbook of thin film technology by Leon I. Maissel and Reinhard Glang, McGraw-Hill Book Company, chapter 7, pp 48-49. "Special pattern-formation techniques". PA1 a) depositing on an electrically insulating substrate a layer of a first conductive material, PA1 b) producing photosensitive resin patterns on the layer of the first conductive material defining the patterns to be etched in said layer, PA1 c) eliminating the areas of the first conductive material layer not covered with resin, PA1 d) depositing an amorphous hydrogenated carbon layer on the structure obtained in c), PA1 e) dissolving the resin patterns in order to eliminate the amorphous carbon deposited on the resin, PA1 f) depositing a layer of a semiconductor on the structure obtained in e), PA1 g) depositing a layer of a first electrical insulant on the semiconductor layer, PA1 h) depositing a layer of a second conductive material on the layer of the first insulant, PA1 i) photoengraving the stack of layers of the second conductive material, the first electrical insulant and the semiconductor in order to fix the dimensions of the transistor and PA1 j) passivating the structure obtained in i) with a second electrical insulant.
As a result of its mechanical characteristics, amorphous hydrogenated carbon, designated a-C:H, is at present mainly used as a protection material (optical components, tools, etc.), but its electrical properties also make it possible to consider its used as a dielectric.
The aforementioned document by Warhue teaches (FIGS. 2 and 11) the obtaining of a-C:H films with high resistivities (10.sup.12 to 13.sup.13 ohms.cm) by using very low gas pressures of .ltoreq.4 Pa (30 mTorrs) and radio-frequency power levels of 10 W. Under these experimental conditions, the a-C:H films are highly stressed and adhere badly to the substrate. There is a risk of the separation of 150 to 200 nm films.
The production of an active matrix for a flat-faced liquid crystal screen having two masking levels, as described in French patent FR-A-2 533 072, makes it necessary to etch a thin metal oxide film supported by a glass substrate in order to reveal the columns of the matrix. This oxide is indium-tin oxide (ITO). This metal oxide etching takes place by the wet route using a solution containing hydrochloric acid and iron perchloride.
The wet route etching speeds are generally considered as very short distance isotropic. However, on large substrates (&gt;1 dm.sup.2), there is a gradient of the etching speed between the peripheral zones and the centre of the substrate. The highest etching speeds are observed on the edges of the substrate. As a result of this phenomenon, in order to obtain a complete etching of the thin metal oxide film, it is necessary to overexpose the peripheral zones to the etching bath.
There can then be a partial etching of the glass and the diffusion of chlorine ions into its volume. Therefore there is a deterioration in the quality of the thus exposed glass surface.
The thin film transistors used in flat-faced screens use as the semiconductor material amorphous hydrogenated silicon, designated a-Si:H. The structure of these transistors leads to the deposition of said silicon directly on the glass. There is then a migration of the chlorine ions diffused in the glass. This chlorine ion migration and the quality of the amorphous silicon-glass interface modify the semiconducting properties of the silicon, leading to a deterioration of the electrical properties and a limited life for the said transistors.
The first problem which the present invention seeks to solve is the reproducible control of the quality of the amorphous silicon-glass interface by proposing a process for the local passivation of the glass substrate. This problem has long existed and has not hitherto been satisfactorily solved.
Problems of the control of the quality of the semiconductor-substrate interface also exist for substrate types other than glass and for semiconductor materials other than amorphous hydrogenated silicon. The invention relates to any local passivation of a random substrate.
EP-A-377 365 describes a local passivation of a substrate by a polymer deposited simultaneously with the erosion of metal oxide patterns using a particular mixture of three gases. This erosion/deposition method leads to thickness inhomogeneities and to a lack of uniformity of the mechanical and electrical characteristics in the polymer layer for substrate surfaces of .gtoreq.1 dm. Thus, said method is not usable for producing large flat-faced display screens.